This application is the National Phase of International Application PCT/JP00/04706 filed Jul. 13, 2000 which designated the U.S. and that International Application was published under PCT Article 21(2) in English.
1. Field of the Invention
The present invention relates to an alternating current detector suitable for detecting an alternating current having a large amplitude and a low frequency.
2. Related Background Art
In vector control for an AC motor, it is required that an alternating current be converted into an AC voltage and there be obtained a voltage waveform with a less phase delay than a current waveform. The conversion of the alternating current into the AC voltage involves the use of a current transformer generally known as CT and of a Hall CT making use of the Hall effect. Of these components the Hall CT is expensive and limited in its application, and therefore the current transformer will hereinafter be described.
FIG. 5 is a circuit diagram for explaining a principle of the current transformer (CT). Referring to FIG. 5, a conductor connected to a detection current source I1 constitutes a one-turn primary winding L1 of the current transformer. A resistance of this conductor is set as a primary winding resistance r1, and these components are connected in series, thereby forming a primary circuit. Further, the current transformer has a secondary winding L2 magnetically coupled to the primary winding L1 via a core C. A resistance of the secondary winding L2 is set as a secondary winding resistance r2, and these components are connected in series, thereby forming a secondary winding circuit of the CT. A voltage detection resistance R is connected to an output terminal of this CT secondary winding circuit, thereby forming a secondary circuit.
Herein, let 1 [T] be the number of turns of the primary winding L1, n2 [T] be the number of turns of the secondary winding L2, i1 be a current of the primary winding L1 which flows in an arrowhead direction, i2 be a current of the secondary winding L2, and, with one end, i.e., the lower end in the Figure, of the voltage detection resistance R being set as a reference potential terminal, V be a voltage at the other end, i.e., the upper end in the Figure, of the voltage detection resistance R as viewed from this reference potential terminal, and a relation in the following formula is established between these parameters.
i1=n2xc2x7i2xe2x80x83xe2x80x83(1)
Further, supposing that a secondary winding resistance r2 is set to zero (0), the voltage v is not induced, and hence a time change of the secondary winding current i2 becomes zero as shown in the following formula.                                           ⅆ                          i              2                                            ⅆ            t                          =        0                            (        2        )            
Accordingly, as shown in the following formula, the secondary winding current i2 is kept constant.                               i          2                =                              i            1                                n            2                                              (        3        )            
In the actual CT, however, there exist the secondary winding resistance r2 and the voltage detection resistance R also called a shunt resistance, and an electromotive force e2 given by the following formula occurs at both ends of the secondary winding n2.                               e          2                =                              (                                          r                2                            +              R                        )                    ·                                    i              1                                      n              2                                                          (        4        )            
This secondary winding resistance r2 and the voltage detection resistance R are factors for the time change of the secondary winding current i2.
Now, let L2 be an inductance of the secondary winding L2, and a time change rate of the secondary winding current is expressed by the following formula:                                           ⅆ                          i              2                                            ⅆ            t                          =                                            e              2                                      L              2                                =                                    1                              L                2                                      ·                          (                                                r                  2                                +                R                            )                        ·                                          i                1                                            n                2                                                                        (        5        )            
Hence, the relation in the formula (3) which has been established immediately after the electrification is lost, and the magnetic flux "PHgr" occurs in the core C.
In the conventional alternating current detector based on the CT, even when the primary current i1 is fixed, the secondary winding current i2 attenuates with an elapse of time, and the magnetic flux "PHgr" occurs in the core C. Hence, there arises a problem, in which an error comes out when converting the detection voltage waveform into the current waveform, and, even if the detection voltage is in the vicinity of zero, the electromotive force for the winding resistance is not zero, or the core is easy to saturate in the large current having a low frequency.
It is a primary object of the present invention, which was devised to solve the problems described above, to provide an alternating current detector capable of highly precisely observing a waveform of a large current having a low frequency with an output of a voltage having a high amplitude.
To accomplish the above object, according to one aspect of the present invention, an alternating current detector comprises a current transformer having a primary winding through which a current to be detected flows, and a secondary winding and an auxiliary winding, magnetically coupled to the primary winding via a core, of which one ends are connected to each other and thus form a common terminal, and an operational amplifier for amplifying a voltage induced in the auxiliary winding, and applying the voltage to the secondary winding with the current assuming such a polarity as to restrain a change in magnetic flux of the core, whereby a voltage corresponding to the current to be detected is induced in the secondary winding.
According to another aspect of the present invention, an alternating current detector comprises a current transformer having a primary winding through which a current to be detected flows, and a secondary winding magnetically coupled to the primary winding via a core, a common terminal constructed of a middle tap provided in the secondary winding, and an operational amplifier, having one winding set as a main winding and the other winding set as an auxiliary winding with the common terminal being the boundary, for amplifying a voltage induced in the auxiliary winding, and applying the amplified voltage to the main winding with the current assuming such a polarity as to restrain a change in magnetic flux of the core, whereby a voltage corresponding to the current to be detected is induced in the main winding.
The common terminal may be connected to a reference potential terminal of the operational amplifier.
The alternating current detector may further comprise a current data reading system for reading the voltage corresponding to the current to be detected. A reference potential of the operational amplifier may be set as an intermediate potential of a voltage of a power source of the current data reading system.